Its like an airport. Some pass through without a pat down and magic wand some people get stopped. Security teams profile you, using police advice and "training". They look for key indicators, such as a padded jacket on a hot day, or very lumpy pockets etc.
In the military, well RAF, we call this combat indicators.
It would be time consuming to check every single person so it would be random checks. Those that want to bring a flare or knife to the game will not understand the random checks or risk being checked. Any that do should be instantly banned for life.
Most trouble is caused between the home and away supporters, prob hence the Lyndhurst checks.
The security company won the contract and have employed lots of new people and as such nonusing trial and error.
Is it right or wrong,? Sadly its the sign of the times we live in.